294 



NATURE 



[July 30, 1891 



copied from my note-book. It was evidently meant to indicate 

 the continuation of the tail towards the nucleus, as seen on 

 subsequent mornings, when farther from the sun. D is the ter- 

 minal edge of the tail, as sharp as the outer limb of the moon, 

 and of fullest strength of lustre. Altogether it formed a rather 

 appalling apparition. Clouds soon obscured it. No farther view 

 was obtained for two or three days, when the end of the tail had 

 assumed the usual misty, indefinite outline. 



The conclusion forced upon my mind was that the comet, 

 having parted with its tail in its rapid turn at the perihelion, was 

 seen in the act of forcing out a new one ahead of itself, in a solid 

 bank of vapour, the front of which might be compared to the 

 wall of water that heads a freshet in a stream. Another re- 

 semblance suggested was that of the solid-looking outline of an 

 np-rolling cumulus cloud. 



I will add hereto a statement made to me at the time by the 

 Rev. Hiram Bingham, a distinguished pioneer missionary to the 

 Gilbert Islands. He saw the comet about a week earlier than 

 myself, from Kaneohe, on the east side of Oahu. Both he and 

 his wife observed waves of prismatic colours running outivard 

 along the brilliant tail. Mr. Bingham is a highly cultivated 

 person, and having commanded the missionary ship for part of 

 two years, is accustomed to lunar and stellar observation. I was 

 led at the time to believe that there was no optical illusion in 

 what he saw. Sereno E. Bishop. 



Honolulu, June 30. 



Copepoda as an Article of Food. 



Prof. Herdman's practical demonstration at the North 

 Cape confirms a theory I have long held, that the Copepoda, 

 which abound in every ocean, sea, and lake, might be largely 

 and advantageously made available for human food. It is well 

 known that the species Calatius finmarchicus, so abundant in 

 our northern seas, forms the chief food of the Greenland whale. 

 Our own immediate coasts abound in this and other equally 

 edible species. During a recent dredging cruise round the Isle 

 of Man, each pull of the tow-net contained thousands of 

 another and larger species of Copepod, Anomalocera patersonii ; 

 and Dr. John Murray has found that a still larger species, EuchcEta 

 norvegica, is plentiful in the lower depths of several Scotch lochs. 



A number of finely-meshed trawls, used off the west coast of 

 Ireland, would, I am convinced, furnish excellent food for 

 starving multitudes in time of need. 



A propos of the distribution of Copepoda, my attention was 

 called a few days ago by the Mayor of Bootle to the filter-beds 

 of the town salt-water baths, which he said were swarming with 

 Entomostraca. The water is supplied direct from the river, 

 and examination showed the presence of Copepoda in enormous 

 quantities, the bulk of them being Eurytemora hirundo, a species 

 only once before taken in Britain, and then in near proximity to 

 Bootle. Probably other filter-beds are equally prolific, and may 

 prove valuable hunting-grounds, the Copepoda undoubtedly 

 acting as scavengers in keeping the water pure from putrefac- 

 tion. I. c. Thompson. 



Liverpool, July 24. 



Meteorological Phenomenon. 



I HAVE received in a letter from a friend residing in Boraston, 

 Shropshire, the following account of a remarkably interesting 

 meteorological phenomenon, which is well worth putting on 

 record : — 



" We had a curious sight from this house yesterday [July 26]. 

 It was a dead calm, but in a field just below the garden, with 

 only one hedge between us and it, the hay was whirled up high 

 into the sky, a column connecting above and below, and in the 

 course of the evening we found great patches of hay raining 

 down all over the surrounding meadows and our garden. It 

 kept falling quite four hours after the aftair. There was not a 

 breath of air stirring as far as we could see, except in that one 

 spot." Francis Galton, 



Refraction through a Prism. 



In such elementary text-books on geometrical optics as I have 



consulted it has always seemed to me that the writers have 



found a difficulty in presenting a precise direct proof of the 



theorem that when a ray is turned out of its course by direct 



NO. II 35. VOL. 44] 



passage through a prism, its deviation is least when its path is 

 symmetrical with regard to the prism. 



May I ask you to consider the simple proof which I inclose, 

 and may I leave it to your judgment whether it is worth while 

 that it should be presented to the notice of teachers in the pages 

 of Nature ? My knowledge of text-books I cannot suppose to 

 be exhaustive, and the arrangement of the proof which I inclose 

 of course may not be any novelty. 



John H. Kirkby. 



Radley College, Abingdon, June 11. 



Minimum Deviation. 

 The problem is to find two rays which, passing directly 

 through a prism near together, have their directions changed 

 by the prism to the same amount — for in the limit, these, when 

 brought into coincidence by change of position of the prism, will 

 mark the course of that ray which suffers minimum deviation 

 (experiment may be appealed to, to show that it is minimum 



and not maximum). Let ABCD be the course of a ray of 

 light through the prism whose vertex is V. At B make the 

 ^ VBC = ^ VCB, then if the ray BC is continued out of the 

 prism on both sides, it is evident that its completion D'C'BA' 

 meets and leaves the faces of the prism at exactly the same 

 angles as the original ray ABCD, only in the opposite direc- 

 tion. Thus the two rays A BCD, A'BC'D' suffer equal deviation, 

 and because the A's VBC, VCB are similar, 



. •. VB- = VC . VC ; 

 and when the rays are so close as to practically render C, C co- 

 incident, we have VB- — VC'-, or-VB = VC when the devia- 

 tion is a minimum,. ?.f. the deviation is a minimum when the 

 course of the ray makes equal angles with the sides of the 

 prism. 



[Oxford men will remember that more than twenty years ago 

 Prof Clifton gave a somewhat similar proof as follows : — 



Since the paths ABCD and D'C'BN are similar, if one is a 

 path of minimum deviation the other must have the same 

 property also. Hence, since light can always travel in the 

 reverse direction along a path, the paths 



ABCD and NBC'D 

 are both paths of minimum deviation. 



But the existence of two such minima is contrary to experi- 

 ment. Hence the paths must be identical, which can only be 

 the case of the angle VBC = VBC = VCB.— Ed.] 



Further Notes on the Anatomy of the Heloderma. 

 Since I published in Nature (vol. xliii. p. 514), " The Poison 

 Apparatus of the Heloderma," there has appeared from the 

 pen of Mr. Boulenger another notable contribution to the 

 anatomy of that genus of reptiles, entitled "Notes on the 

 Osteology of Heloderma horridum and H. suspectum, with 

 Remarks on the Systematic Position of the HelodermatidiE a.nd on 

 the Vertebras of the Lacertilia," ( P. Z.S., January 20, 1891). That 

 paper is especially useful, inasmuch as it critically compares the 

 vertebral columns of the two species of Lizards under consider- 

 ation—a comparison which, up to the time of the appearance 

 of Mr. Boulenger's paper, had not been made. To briefly 

 recapitulate his points, Boulenger finds differences in the form 

 of the premaxillaries of the two species, and in the number of 

 teeth supported by those bones. He finds palatine and pterygoid 

 teeth constantly absent in H. suspectwn but present in H. 

 horridum — a very remarkable fact. A small azygous ossification 

 was found in the cartilage of the mandibular symphysis of 

 H. horridum, "apparently the homologue of the symphysial 

 (mento-meckelian) bones of most tailless Batrachians." This 



